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Abstract:

A high-voltage isolation transformer utilizing balun cores for
transferring an alternating current signal from a first circuit to an
output circuit, and a high voltage generator, such as a Cockcroft-Walton
multiplier to apply a high direct current voltage bias with a high degree
of voltage isolation between the first circuit and the output circuit.
Multiple balun core transformers can be used in order to reduce the
voltage rise between each individual transformer.

Claims:

1. A transformer circuit with high voltage isolation comprising:a) a first
circuit configured to carry an alternating current signal at a low
voltage;b) a balun core having at least two holes through the balun
core;c) the first circuit looped through the at least two holes in the
balun core at least one time, wherein the first circuit acts as a primary
winding of the transformer circuit;d) an output circuit looped through
the at least two holes in the balun core at least one time, wherein the
output circuit acts as a secondary winding of the transformer circuit,
the output circuit being electrically coupled to a high voltage direct
current signal source to provide a direct current bias to the alternating
current signal with a high level of voltage isolation between the first
circuit and the output circuit.

2. A transformer circuit with high voltage isolation, as in claim 1,
wherein the high voltage direct current signal source is a high voltage
generator configured to provide the high voltage direct current signal
that is connected to the output circuit.

3. A transformer circuit with high voltage isolation, as in claim 2,
wherein the high voltage generator is a Cockcroft-Walton multiplier.

4. A transformer circuit with high voltage isolation, as in claim 1,
further comprising insulation configured to substantially cover at least
one of the first circuit and the output circuit, wherein the insulation
is fluorinated ethylene propylene.

5. A transformer circuit with high voltage isolation, as in claim 1,
further comprising:a thermionic cathode of an x-ray tube operable to be
electrically coupled to the output circuit to act as a load on the output
circuit.

6. A transformer circuit with high voltage isolation, as in claim 5,
wherein the x-ray tube is used in an x-ray fluorescence analyzer.

7. A transformer circuit with high voltage isolation comprising:a) a first
circuit operable to carry an alternating current signal at a low
voltage;b) a first balun core having at least two holes through the
core;c) the first circuit looped through the at least two holes in the
first balun core at least one time, as a primary winding of a first
transformer;d) an intermediate circuit looped through the at least two
holes in the first balun core at least one time, as a secondary winding
of the first transformer, wherein the intermediate circuit is coupled to
a mid-level on a direct current signal source;e) a second balun core
having at least two holes through the core;f) the intermediate circuit
looped through the at least two holes in the second balun core at least
one time, as a primary winding of a second transformer;g) an output
circuit looped through the at least two holes of the second balun core at
least one time, as a secondary winding of the second transformer, wherein
the output circuit is coupled to a high voltage direct current signal
source to provide a direct current bias to the alternating current signal
with a high level of voltage isolation between the first circuit and the
output circuit.

8. A transformer circuit with high voltage isolation, as in claim 7,
wherein the direct current signal source is a Cockcroft-Walton multiplier
configured to provide a mid-level direct current signal to the
intermediate circuit and a high voltage direct current signal to the
output circuit.

9. A transformer circuit with high voltage isolation, as in claim 8,
wherein the Cockcroft-Walton multiplier mid-level voltage direct current
signal is connected to the intermediate circuit through a circuit
isolation device.

10. A transformer circuit with high voltage isolation, as in claim 9,
wherein the circuit isolation device is a metal-oxide varistor.

11. A transformer circuit with high voltage isolation, as in claim 7,
further comprising insulation configured to substantially cover at least
one of the first circuit, the intermediate circuit, and the output
circuit, wherein the insulation is formed of fluorinated ethylene
propylene.

12. A transformer circuit with high voltage isolation, as in claim 7,
further comprising a thermionic cathode of an x-ray tube coupled to the
output circuit to act as a load on the output circuit.

13. A transformer circuit with high voltage isolation, as in claim 12,
wherein the x-ray tube is used in an x-ray fluorescence analyzer.

14. A transformer circuit with high voltage isolation comprising:a) a
sequence of at least three balun cores, in which there is a first balun
core, at least one intermediate balun core, and a final balun core,
wherein each of the balun cores includes at least two holes through each
core;b) a sequence of circuits, in which there is a first circuit, at
least two intermediate circuits, and an output circuit, and the total
number of circuits is equal to the total number of balun cores plus
one;c) the first circuit operable to carry an alternating current signal
at a low voltage and the first circuit looped through the at least two
holes in the first balun core at least one time, as a primary winding of
a first transformer;d) a first intermediate circuit looped through the at
least two holes in the first balun core, as a secondary winding of the
first transformer, and looped through the at least two holes of a second
balun core at least one time, as a primary winding of a second
transformer;e) wherein each of the intermediate balun cores is connected
to a succeeding balun core by an intermediate circuit that is looped
through the at least two holes in each of the intermediate balun cores at
least one time;f) wherein the final balun core is connected to the
previous, intermediate balun core by an intermediate circuit that is
looped through each of the at least two holes at least one time in the
final and intermediate balun cores;g) wherein the output circuit is
looped through the at least two holes in the final balun core at least
one time;h) a means for providing a sequence of high voltage access
points for the circuits, wherein:1. a number of high voltage access
points is equal to a number of intermediate circuits plus one;2. a
highest high voltage access point has a voltage higher than any other
high voltage access point;3. a first high voltage access point has a
voltage that is approximately equal to a voltage of the highest high
voltage access point voltage divided by the number of high voltage access
points;4. a voltage at each successive high voltage access point is
greater than a voltage at a previous high voltage access point; andi) the
first intermediate circuit is connected to the first high voltage access
point, and each succeeding intermediate circuit is connected to a next
corresponding high voltage access point, such that there is an increase
in voltage at each successive intermediate circuit, and the highest high
voltage access point is connected to the output circuit to provide a
direct current bias to the alternating current signal at the output
circuit with a high level of voltage isolation between the first circuit
and the output circuit.

15. A transformer circuit with high voltage isolation, as in claim 14,
wherein a difference in voltage between each high voltage access point
and the succeeding high voltage access point is approximately equal to
the voltage of the highest high voltage access point divided by the
number of high voltage access points.

16. A transformer circuit with high voltage isolation, as in claim 14,
wherein:a) the means for providing the sequence of high voltage access
points is a Cockcroft-Walton multiplier; andb) each intermediate circuit
is connected to the Cockcroft-Walton multiplier through a circuit
isolation device.

17. A transformer circuit with high voltage isolation, as in claim 14,
wherein the circuit isolation device is a metal-oxide varistor.

18. A transformer circuit with high voltage isolation, as in claim 14,
further comprising insulation configured to substantially cover at least
one of the first circuit, the at least one intermediate circuit, and the
output circuit, wherein the insulation is formed of fluorinated ethylene
propylene.

19. A transformer circuit with high voltage isolation, as in claim 14,
wherein a load on the output circuit is a thermionic cathode of an x-ray
tube.

20. A transformer circuit with high voltage isolation, as in claim 14,
wherein the x-ray tube is used in an x-ray fluorescence spectrometer.

Description:

[0002]It is difficult, in an electrical transformer, to insulate a very
low voltage primary circuit from a very high voltage secondary circuit
due to the voltage difference between the two circuits. Another problem
with some high voltage isolation transformers is the generation of
significant electromagnetic waves at an amplitude and frequency that may
interfere with sensitive electronic components. For example, some
handheld x-ray fluorescence (XRF) spectrometers require high voltage
isolation transformers to provide a small AC signal at a large negative
DC potential for the thermionic cathode of an x-ray tube. Electromagnetic
waves from these transformers can interfere with an x-ray signal received
by an x-ray detector in the XRF spectrometer.

[0003]Optimal operation of a transformer is typically at the transformer's
resonant frequency. In XRF analyzers using transformers with torroidal
shaped cores, electromagnetic waves emitted at the core's resonant
frequency may significantly interfere with the operation of the x-ray
detector. In addition, the shape of a toroidal transformer can result in
a high level of electromagnetic interference (EMI). Shielding and circuit
design are often used to mitigate the electromagnetic interference of the
detector, but eliminating this interference with circuit design and
shielding, especially in the small space available in a relatively small
handheld XRF spectrometer, can be difficult.

[0004]A toroid shaped core, made of ferromagnetic material, may be used in
a high voltage isolation transformer. For example, in handheld XRF
spectrometers, the primary windings of the transformer have a relatively
low voltage, typically around 10 volts rms AC. The secondary windings
carry an alternating current, induced by the AC signal on the primary
windings. The secondary windings also have a very large bias voltage of
around negative 50,000 volts compared to the primary windings. This bias
voltage is generated primarily by a high voltage power supply that is
used to apply the bias voltage to the secondary windings. It is very
difficult to effectively insulate circuits with such a large voltage
difference.

[0005]High voltage isolation transformers having a toroid shaped core can
have stringent design and manufacturing requirements. To isolate the two
widely disparate voltages, of the primary and secondary windings, thick
insulation is typically applied to the transformer core, the wire, or
both wire and core. Insulation is used that can maintain its integrity
and be free of cracks in order to avoid current leakage between the
primary and secondary windings. If the bulk of the insulation is on the
core, the insulation can crack due to thermal expansion caused by the
heating and cooling of the core. One cause of the insulation cracks,
during these temperature fluctuations, is a mismatch of the coefficient
of thermal expansion (CTE) of the core compared with the CTE of the
insulation. Creating this match can be a difficult design challenge.
Applying crack-free insulation is often a difficult manufacturing
challenge. Thicker insulation can be more difficult to manufacture,
without insulation defects, than thinner insulation.

SUMMARY OF THE INVENTION

[0006]A transformer circuit with high voltage isolation is disclosed. The
transformer circuit includes a first circuit configured to carry an
alternating current signal at a low voltage level. The first circuit is
looped through at least two holes in a balun core to act as a primary
winding of the transformer circuit. An output circuit is looped through
at least two holes in the balun core to act as a secondary winding of the
transformer circuit. The output circuit is electrically coupled to a high
voltage direct current signal source to provide a direct current bias to
the alternating current signal with a high level of voltage isolation
between the first circuit and the output circuit. More than one balun
core may be used in series to allow a more gradual increase in voltage.

[0007]There has thus been outlined, rather broadly, various features of
the invention so that the detailed description thereof that follows may
be better understood, and so that the present contribution to the art may
be better appreciated. Other features of the present invention will
become clearer from the following detailed description of the invention,
taken together with the accompanying claims, or may be learned by the
practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 shows a high voltage isolation transformer in which the
transformer is split into two cores.

[0009]FIG. 2 is a balun core for use in a transformer.

[0010]FIG. 3 shows a high voltage isolation transformer in which the
transformer is split into at least three cores.

[0011]FIG. 4 shows a high voltage isolation transformer with a single
balun core.

[0012]FIG. 5 shows the high-voltage connection point on an intermediate
circuit of a high voltage isolation transformer.

[0013]FIG. 6 shows a Cockcroft-Walton multiplier circuit connected to a
high voltage isolation transformer in which the transformer is split into
two cores.

DETAILED DESCRIPTION

Definitions:

[0014]A balun transformer core, balun core, or balun, as defined in this
application is a transformer core with at least two holes, as shown in
one exemplary embodiment illustrated in FIG. 2, indicated generally at
200. The balun has a top surface 202, a bottom surface 204 and a side
surface 203. Normally the shape of the top and bottom surfaces is
circular or elliptical, but they can also be square, triangle,
rectangular, or other shape. The benefit of a circular or elliptical
shape of these surfaces, as shown in FIG. 2, is that it allows a smooth
side surface 203 and fewer edges where other components and insulation
can be cut or where corona stress may occur. The balun illustrated in
FIG. 2 includes two holes 201 which extend from the top surface 202
through the balun to the bottom surface 204. The balun may include more
than two holes. The length L of the balun is normally longer than the
hole diameter D, to allow more distance for electrically coupling a wire
that passes through the hole to the core. Usually the length L is at
least twice the hole diameter D. Balun cores may be found and purchased
under the name "Wideband Multi-Aperture Balun Cores".

[0015]AC and DC, as used in this application have their normal meanings of
Alternating Current and Direct Current. EMI is the acronym for
Electromagnetic Interference and has its usual definition of
electromagnetic interference with the proper operation of an electronic
circuit.

[0016]XRF is an acronym for x-ray fluorescence and is the emission or
fluorescence of x-rays from a material that has been excited by
bombarding the material with X-rays or gamma rays. XRF spectrometers can
provide an x-ray source for bombarding a sample with x-rays and also have
a detector for quantifying the amount and energy of x-rays fluoresced by
the sample. XRF spectrometers can be used for analyzing what elements a
material is made of.

[0017]FEP is an acronym for fluorinated ethylene propylene. FEP is a type
of insulating material with a high dielectric strength.

Description

[0018]Reference will now be made to the exemplary embodiments illustrated
in the drawings, and specific language will be used herein to describe
the same. It will nevertheless be understood that no limitation of the
scope of the invention is thereby intended. Alterations and further
modifications of the inventive features illustrated herein, and
additional applications of the principles of the inventions as
illustrated herein, which would occur to one skilled in the relevant art
and having possession of this disclosure, are to be considered within the
scope of the invention.

[0019]A high voltage isolation transformer that interferes less with
sensitive electromagnetic components is relatively easy to manufacture,
and is reliable. This may be accomplished by using A transformer split
into multiple transformer cores can be used to reduce high voltage
isolation problems. The use of multiple cores also reduces the DC stress
across each core and reduces the required amount of insulation at each
core. Transformer cores with thinner insulation are easier to
manufacture. Further improvement is realized with the use of a balun core
or multiple balun cores as the transformer cores. Balun cores have a much
wider bandpass than toroid cores, and can thus be driven at a high enough
frequency to be outside the sensitivity range of the X-ray detector
electronics. Use of a balun core instead of a toroid shape, or many other
core shapes, results in a reliable, easy to manufacture design, and less
EMI. The EMI emitted by the balun core is also easier to shield because
this EMI is more directional than the EMI emitted by a toroid core.

[0020]One purpose of the transformer, or transformers in a multiple
transformer design, can be to transfer an AC signal from one circuit to
another circuit. A high voltage generator can be used to create a large
bias voltage between the circuits. The high voltage generator, such as a
Cockcroft-Walton Multiplier, can generate a high voltage from a low
voltage, in a compact space. Some high voltage generators, such as a
Cockcroft-Walton Multiplier, can provide a voltage rise in steps.
Circuits external to such high voltage generators can connect at each of
these different voltage steps, providing a series of high voltage access
points of increasing voltage. The Cockcroft-Walton Multiplier is
especially valuable in handheld, battery-operated XRF spectrometers,
because of limited space and limited available power in this equipment.

[0021]For example, if the bias voltage rise occurs across two
transformers, only half of the total voltage rise occurs at each
transformer, thus the insulation thickness required at each transformer
is significantly less than what is required for a single transformer. The
voltage rise across each transformer may be created by using a
Cockcroft-Walton multiplier. The primary windings of the first
transformer, called a first circuit, carry a low voltage AC signal. The
secondary windings of the first transformer, called an intermediate
circuit, are also the primary windings of the second transformer. This
intermediate circuit is attached to a mid-level voltage point on the high
voltage generator. The secondary windings of the second transformer,
called the output circuit, are connected to the highest voltage point on
the high voltage generator. The output circuit provides an AC signal at a
high bias voltage to the load. The high DC bias can be either a large
negative bias or a large positive bias. Generally, the intermediate
windings of the transformers are limited to one turn, and the majority of
the HV insulation is provided on these windings. For example, the
intermediate winding can be made of wire with thick FEP insulation
approximately 0.1'' in diameter that is capable of insulating more than
30 kV.

[0022]For example, in a handheld XRF spectrometer, the first circuit
carries an AC signal at around 10 volts rms. The AC signal is induced in
the intermediate circuit and the connection to the high voltage generator
enables a DC bias to be maintained between the first circuit and the
intermediate circuit at around negative 25,000 volts DC. The AC signal is
then induced in the output circuit. A separate connection to the high
voltage generator enables a bias between the intermediate circuit and the
output circuit at around negative 25,000 volts to be maintained, for a
total bias between the first circuit and the output circuit of around
negative 50,000 volts DC.

[0023]With two transformer cores, the voltage rise at each transformer is
only 25,000 volts, allowing insulation rated at 30,000 volts to be used.
Having the two cores allows the use of wire with an insulation rating of
30,000 volts to be used while achieving 50,000 volts of isolation between
the first circuit and the output circuit and enabling a small AC signal,
such as the 10 volt rms signal used in the example above, to be applied
to the highly biased signal. The number of primary, secondary,
intermediary, or output windings may be changed if a lower or higher AC
signal is desired.

[0024]In an XRF spectrometer, the thermionic cathode normally operates at
a very large negative DC potential relative to the anode. For example,
the anode may approximate ground voltage and the cathode may be about
negative 50,000 volts. This large negative potential results in
acceleration of electrons from the cathode to the anode. A small AC
signal, typically less than 10 volts AC rms, can also be applied to the
cathode. The AC signal is used to heat up the cathode for improved
electron emission.

[0025]More transformers may be used in series, allowing an even more
gradual increase in voltage at each stage, or a higher overall voltage
rise with the same increase in voltage at each stage, while maintaining
the original AC signal. Different high voltage generator access points,
with each successive access point higher in voltage than the previous,
are used to connect to each of the intermediate circuits and to the
output circuit. Each intermediate circuit is the secondary winding of the
previous transformer and the primary winding of the succeeding
transformer. The first intermediate circuit is connected to the lowest
high voltage generator access point. The next intermediate circuit is
connected to a higher, high voltage generator access point. Each
successive intermediate circuit is connected to a high voltage generator
access point having a greater voltage than the previous intermediate
circuit, until, at the end, the output circuit is connected to the
highest high voltage generator access point. A series of transformers in
high voltage isolation applications can provide beneficial space savings.
The high voltage generator is typically relatively long. A chain of
transformers is also usually relatively long and can conveniently extend
through space in the equipment adjacent to the high voltage generator.

[0026]The transformer core is a means for inducing alternating current in
the secondary windings of a transformer. The core aids in efficient
transfer of the electrical signal from the primary winding to the
secondary winding in a transformer. Many cores shapes are available and
well known in the art, such as pot, planar, economical flat design (EFD),
ER, EP, toroid, bar, rod, C, U, E, and F shaped cores. In one embodiment
of the present invention, two balun cores are used as the transformer
core rather than a toroid or other shaped core. A balun core is described
in U.S. Pat. No. 7,319,435, incorporated herein by reference.

[0027]A balun core can have a higher resonant frequency than a toroidal
shaped core, allowing a higher drive frequency and thus smaller balun
cores to be used. In some XRF analyzers, the toroidal transformers are
typically operated at their resonant frequency--around 100 kHz. In
experimental XRF analyzers, the balun core transformers are operated at
their resonant frequency, which can be around 1 MHz or higher. Presently,
the experimental XRF analyzers with balun cores are operated at 2.5 MHz.
The higher resonant frequency of the balun core can cause significantly
less XRF detector interference.

[0028]A transformer having balun cores can provide reduced leakage
inductance and better coupling as compared with other core types. For
example, a torroidal shaped core can produce lower frequency EMI due to
the lower resonant frequency of the toroid core. This lower frequency EMI
can have a more adverse effect on XRF detectors than the higher frequency
EMI that occurs at the higher resonant frequency of the balun core. The
balun core can have a maximum bandwidth and lower power loss at high
frequency, allowing operation of the balun core transformers in a range
that produces EMI at a frequency that is less detrimental to an XRF
detector. The balun may be made of any standard transformer core
material, such as powdered iron, steel, or ferrite, depending on the
frequency of operation. Other materials may also be used. Core material
affects performance and should be a design consideration. Presently,
ferrite is the preferred core material. The actual material can be
selected to be suitable to the specific application and is not critical
to the present invention.

[0029]FIG. 4 shows one exemplary embodiment of a high voltage isolation
transformer, indicated generally at 400. An electrical circuit 109,
called a first circuit, can carry an alternating current at low voltage.
The first circuit is looped in through one hole and out through the other
hole of the balun core 404 and is the primary winding. The first circuit
can be looped one time or many times. An AC signal is induced in a
secondary winding 112. The secondary winding 112, or output circuit, can
carry an AC signal at a relatively high DC bias voltage to a load 114.
The high voltage isolation is generally accomplished by using a single
turn secondary with sufficient high voltage insulation to withstand the
designed voltage stress per stage. A high voltage generator 401 can
provide a very high voltage bias at access point 402 and can be connected
to the output circuit 112 through connection means 403. The connector, in
this and other embodiments, can be any standard electrical wire with
appropriately rated insulation. The connections between wires or circuits
can be any standard high-voltage electrical connection. Solder is
preferred. In this and other embodiments, the output circuit can be the
cathode of an x-ray tube, or some other circuit that uses an alternating
current at a high DC bias voltage.

[0030]The high voltage generator in the embodiment above and in later
described embodiments may be a Cockcroft-Walton (CW) multiplier. This is
a type of voltage multiplier that is used to convert alternating current
or pulsing DC electrical power from a low voltage level to a higher DC
voltage level. It is comprised of a voltage multiplier ladder network of
capacitors and diodes to generate high voltages. The CW multiplier is
well known in the art. A more detailed description is provided below with
reference to FIG. 6.

[0031]FIG. 1 shows one exemplary embodiment of a high voltage isolation
transformer, which is split into two transformer cores, and is indicated
generally at 100. An electrical circuit 109, called a first circuit,
carries an alternating current at a relatively low voltage, such as
around 10 volts. This first circuit is looped in through one hole and out
through the other hole of the first balun core 107 and acts as the
primary winding of the first balun core 107. The first circuit can be
looped one time or many times. An AC signal is induced in a secondary
winding 110 of the first balun core 107. This first balun core secondary
winding 110 acts as an intermediate circuit and is the primary winding
for a second balun core 108. The intermediate circuit 110 induces an AC
signal in an output circuit 112, which acts as a secondary winding, or
output circuit for the second balun core 108. The intermediate circuit
110 can be looped through the first balun core 107 once or multiple
times, although it usually consists of one turn of high voltage insulated
wire. The intermediate circuit 110 can also be looped through the second
balun core 108 once or multiple times. The output circuit 112 may be
looped through the second balun core 108 once or multiple times. Output
circuit 112 may be connected to load 114. This load may be a thermionic
cathode in an x-ray tube. The x-ray tube may be used in an XRF
spectrophotometer. The ratio of input to output voltage can be modified
by adjusting the ratio of the number of turns of the first primary to the
number of turns of the last secondary. This is useful, for instance, in
matching the drive electronics rms voltage to the voltage required by the
filament of the X-ray tube.

[0032]A high voltage generator 101, with a mid-level voltage access point
105 and a high-level voltage access point 106, can provide a high DC
voltage bias. The mid-level voltage access point 105 can be connected to
an optional circuit isolation means 102 via wire 103.

[0033]A circuit isolation means 102 is used in the circuit between the
high voltage access points 106 of the high voltage generator and the
intermediate circuit 110. The circuit isolation means may be a resistor,
a metal-oxide varistor, or a spark gap or other similar device. The
circuit isolation means isolates radio frequency signals in the
transformer network from the high voltage generator. The circuit
isolation means also creates a bias voltage reference for the
intermediate circuit without creating current path between the high
voltage generator and the intermediate circuit.

[0034]The circuit isolation means 102 can be connected to the intermediate
circuit 110 via wire 104. However, the high voltage isolation transformer
can function without the circuit isolation means 102. The circuit
isolation means 102 is optional in this and other embodiments described
later. If circuit isolation means 102 is not used, then wire 103 is
connected to wire 104 or wires 103 and 104 are one continuous wire. With
or without the circuit isolation means 102, the mid-level access point
105 provides a voltage, that may be approximately half of the voltage at
access point 106, to the intermediate circuit 110. High-level voltage
access point 106 is connected to the output circuit 112 via wire 111,
resistor R and wire 113. Although a resistor R is normally used, the
circuit can function without this resistor. The access point 106 can
provide a very high voltage bias for the output circuit 112.

[0035]FIG. 3 shows a high voltage isolation transformer, which is split
into at least three transformer cores, and is indicated generally at 300.
An electrical circuit 109, called a first circuit, carries an alternating
current at a relatively low voltage. This first circuit is looped in
through one hole and out through the other hole of a first balun core 107
and is the primary winding of the first balun core 107. The first circuit
can be looped one time or many times. An AC signal is induced in a
secondary winding 110 of the first balun core 107. This first balun core
secondary winding 110 is an intermediate circuit and is the primary
winding for a second balun core 309. The intermediate circuit 110 induces
an AC signal in a second intermediate circuit 302, which is a secondary
winding for the second balun core 309. The second intermediate circuit
302 is a primary winding for a third balun core 310, and induces an AC
signal in circuit 303. Circuit 303 may be an output circuit or may be
another intermediate circuit. This same configuration can continue, with
more balun cores and an intermediate circuit looped between each pair of
balun cores in the sequence. The circuit exiting the final balun core is
the output circuit.

[0036]In deciding how many balun cores to be used, the possible benefit of
a smaller voltage difference between adjacent circuits can be balanced
against the possible challenges of a longer transformer and overall power
loss. Having more balun cores, with a smaller voltage difference between
adjacent circuits, allows reduced insulation to be used on the wires. A
chain of more balun cores, however, requires more space. Also, there is a
power loss, between the primary and secondary windings, across each
successive balun core. The possible benefit of reduced insulation can be
weighed against possible disadvantages of a longer chain of balun cores
and the power loss across each balun core.

[0037]Normally, only one intermediate circuit connects two balun cores,
but more may be used. Any of the circuits may loop through a balun core
once or many times, depending on the desired amplitude of the AC signal
at the output circuit relative to the first circuit 109.

[0038]A high voltage generator 308, with multiple voltage access points
provides a high DC voltage bias to the AC signal. Each successive high
voltage access point in the series, moving from left to right across FIG.
3, is a higher voltage than the previous access point. Typically, there
would be an approximately equal voltage difference between any access
point and the preceding or succeeding access point. In this exemplary
embodiment, access point 304 is the lowest voltage, 305 is the next
highest, and 306 is the next highest. If 306 is the last access point, as
shown in this embodiment, then it would be the highest voltage access
point. If 306 is not the last access point, then there can be a
subsequent, higher voltage access point. The final access point is
typically the highest desired voltage. If the change in voltage at each
balun core is substantially equal, then the approximate difference in
voltage between any two access points is equal to the highest voltage
access point divided by the total number of access points. Alternatively,
one or more balun cores may provide a greater change in voltage than
other cores in the series.

[0039]All voltage access points, except the final access point, may be
connected to an optional circuit isolation means 102 (described above)
with a wire 103. The circuit isolation means 102 can be connected to the
transformer winding by another wire 104. In FIG. 3, if the circuit 303 is
an output circuit, then 311 is a resistor. If circuit 303 is an
intermediate circuit, 303 is a circuit isolation means, such as a
metal-oxide varistor.

[0040]The high voltage isolation transformer 300 enables a large DC bias
to be applied to an AC signal while maintaining high voltage isolation
between the first circuit 109 and the output circuit 306. Implementing
the circuit in steps allows a lower change in bias between the balun
cores, thereby enabling thinner insulation to be used. The use of thinner
insulation reduces costs and decreases the size of the overall circuit.

[0041]High voltage isolation transformers are relatively easy to make. The
balun cores may be purchased from numerous sources. Wire can be selected
to have a proper insulation rating for the planned voltage difference
between the primary winding and the secondary winding.

[0042]In one exemplary embodiment having a two balun-core isolation
transformer, the intermediate circuit can include most of the insulation.
This intermediate circuit may only be wound once through each balun core,
due to the thickness of this insulation. Fluorinated ethylene propylene
(FEP) can be used as insulation for the intermediate circuit. FEP can
also be used as insulation for the other circuits. Alternatively, another
material may be used.

[0043]FIG. 5 illustrates an example of a connection point on an
intermediate circuit 501 and is shown generally at 500. In one
embodiment, the connection 501 can be made from any intermediate circuit
503 to the high voltage generator 502, at a mid point between the two
balun cores on each end of the intermediate circuit. In other words, it
is preferred that distance L1 is approximately equal to distance L2 to
maximize the distance from the insulation opening to the balun core. This
is especially important if the bulk of the insulation is on the
intermediate circuit 503 and a reduced amount of insulation is located on
the primary winding 504 of the previous balun core 506 and the secondary
winding 505 of the succeeding balun core 507. With minimal insulation on
the primary winding 504 of the previous balun core 506, the balun core
voltage will approximate the voltage of the primary winding 504 of that
core. With minimal insulation on the secondary winding 505 of the
succeeding balun core 507, the succeeding balun core voltage will
approximate the voltage of the secondary winding 505 of that core. To
avoid current flow along the surface of the insulation, from the opening
in the intermediate circuit 501 to either balun core 506 or 507, the
distance L1 or L2 in centimeters should be approximately equal to 0.00005
multiplied by the voltage potential between the intermediate circuit 503
and the balun core. For example, if there is a voltage difference of
25,000 volts between the intermediate circuit 503 and the first balun
core 506, then the distance between the connection point 501 and the
first balun core 506, distance L1, should be approximately 1.25
centimeters.

[0044]FIG. 6 shows an example of a high voltage isolation transformer
connected to a Cockcroft-Walton multiplier, and is indicated generally at
600. An AC supply can provide alternating current to the Cockcroft-Walton
multiplier. Capacitors C1 through C12 are shown along with diodes D1
through D12 and access points A1 through A6. The amplitude and frequency
of the alternating current, the size and type of diodes and capacitors
may be selected as needed by a particular design to provide the desired
DC bias to the AC signal in the transformer circuit with a high level of
voltage isolation. Two consecutive capacitors and two consecutive diodes,
for example, capacitors C1 and C2 and diodes D1 and D2 comprise a
Cockcroft-Walton multiplier stage. Six Cockcroft-Walton multiplier stages
are shown in FIG. 6. More stages may be added to further increase the
voltage. Connection to the transformer circuits may be made at any of the
access points.

[0045]It is to be understood that the above-referenced arrangements are
only illustrative of the application for the principles of the present
invention. Numerous modifications and alternative arrangements can be
devised without departing from the spirit and scope of the present
invention. While the present invention has been shown in the drawings and
fully described above with particularity and detail in connection with
what is presently deemed to be the most practical and preferred
embodiment(s) of the invention, it will be apparent to those of ordinary
skill in the art that numerous modifications can be made without
departing from the principles and concepts of the invention as set forth
herein.

Patent applications by David J. Caruso, Groton, MA US

Patent applications by MOXTEK, INC.

Patent applications in class INCLUDING A TRANSFORMER OR AN INDUCTOR

Patent applications in all subclasses INCLUDING A TRANSFORMER OR AN INDUCTOR